When a molecular ion generated from a sample molecule is made to move in a gas medium under the effect of an electric field, the ion moves at a speed proportional to its mobility which is determined by the intensity of the electric field, size of the molecule and other factors. Ion mobility spectrometry (IMS) is a measurement method in which this mobility is utilized for an analysis of sample molecules. FIG. 5 is a schematic configuration diagram of a conventional and common type of ion mobility spectrometer (for example, see Patent Literature 1).
This ion mobility spectrometer includes: an ion source 1 for ionizing component molecules in a sample; a drift region 5 which is provided, for example, within a cylindrical housing (not shown), for measuring the ion mobility; and a detector 6 for detecting ions which have traveled through the drift region 5. Additionally, in order to send the ions generated in the ion source 1 into the drift region 5 in a pulsed form during a limited short time width, a shutter gate 3 is provided at an entrance of the drift region 5. The atmosphere inside the housing is maintained at atmospheric pressure or low vacuum of approximately 100 [Pa]. A uniform electric field having a downward potential gradient (for drifting ions) in the moving direction of the ions (in FIG. 5, the Z-direction) is formed within the drift region 5 by DC voltages respectively applied to a number of ring-shaped electrodes 2a included in a drift-electrode group 2 arranged within the drift region 5. A flow of neutral gas is formed in the opposite direction to the direction of the drift motion by the electric field. In order to reduce an image current induced in a detection electrode of the detector 6 by ions drifting with drift motion, the electrode 2a disposed at the final stage preceding the detector 6 employs a grid-type (mesh type) electrode.
The ions generated in the ion source 1 are temporarily blocked by the shutter gate 3. The shutter gate 3 is subsequently opened for a short period of time, whereupon the ions in a packet-like form are introduced into the drift region 5. Colliding with the counterflowing gas within the drift region 5, the introduced ions are driven forward by the electric field. Those ions are temporally separated according to their ion mobilities, which depend on the size, steric structure, electric charge and other properties of the individual ions. Accordingly, ions with different ion mobilities reach the detector 6 demonstrating certain intervals of time. If the electric field within the drift region 5 is uniform, the collision cross-section between an ion and the counterflowing gas can be estimated from the drift time required for the ion to pass through the drift region 5.
The capability to separate a certain kind of ions originating from a sample molecule can be evaluated by the resolving power R calculated by the following formula (1):R=Td/ΔT  (1)where Td is the drift time required for the ion to travel in the drift region 5 and ΔT is the pulse width (temporal spread) of the ions at the time when the ions are detected in the detector 6.
A high-resolution ion mobility spectrometer with high resolving power R is required for separating molecules having molecular weights close to each other or for separating molecules having the same molecular weight but different molecular structures (structural isomers). For increasing the resolving power R, the drift time Td should be increased or the ion pulse width ΔT should be decreased, as is evident from the formula (1). To decrease the ion pulse width ΔT, the shutter open time should be shortened. However, shortening the shutter open time decreases the amount of ions passing through the shutter and causes the sensitivity to deteriorate. Thus, there is a limit in shortening the shutter open time if a certain level of sensitivity is demanded. In addition, even ions having the same ion mobility spread in the front-back direction while travelling through the drift region, due to the diffusion (spatial spread of molecules due to their random motion), dispersion (spatial spread of molecules during their motion in a fluid), or other actions. Thus, there is also a certain lower limit of the ion pulse width ΔT even if the shutter open time is shortened. In view of the above, an effective method for increasing the resolving power in an ion mobility spectrometer is to increase the length of the drift region 5, i.e., the drift length L.
However, in the ion mobility spectrometer, it is necessary to close the shutter gate 3 until all ions introduced in the drift region 5 completely pass through the drift region 5, in order to avoid the situation in which ions with high drift speeds overlap ions with low drift speeds during the measurement. In view of this, if the drift time Td is increased due to the increase of the drift length L as mentioned above, the waiting time also needs to be increased, which is the period of time from a time point when the shutter gate 3 is opened to a time point when the shutter gate 3 is next opened after the previous opening. In other words, an operation period of the shutter gate becomes longer, which results in a decrease in the rate at which the measurement for the ion mobility spectrum can be performed per one second (sampling rate).
Non Patent Literature 1 discloses, for example, that a high-resolution ion mobility spectrometer including a drift tube with a length of 63 cm is used to perform isomer separation on silicon clusters. A typical ion mobility spectrum thereby measured has a significantly long drift time of approximately 100 msec. In this case, the sampling rate is 10 Hz. In a case where the ion mobility spectrometer is used as a detector of a liquid chromatograph (LC) to analyze components in a sample which are sequentially eluted from a column of the LC, a decrease in the sampling rate results in a longer time interval between data points in the chromatogram. This may possibly prevent a peak from being appropriately captured. In an extreme case, some specific ion may be omitted from detection.